The high burn-up of fuel has recently been developed for improving the economy of nuclear power generation. An example of such a fuel assembly for high burn-up will be described with reference to FIG. 16. FIG. 16(A) is an elevation view showing a fuel assembly with parts partially sectioned; FIG. 16(B) is a sectional view taken along line B--B of FIG. 16(A); and FIG. 16(C) is a sectional view taken along line C--C of FIG. 16(A).
Referring to FIG. 16(A), the fuel assembly 1 comprises long fuel rods 2, short fuel rods 3 and large water rods 6, all of which are bundled in the form of a square lattice with spacers 8 and secured to an upper tie plate 4 and a lower tie plate 5 to form a bundle of fuel rods, which is in turn enclosed by a channel box 7. Furthermore, outer springs 9 are interposed between the long fuel rods 2 and the upper tie plates 4.
** 1 The construction of the long fuel rods 2 and the short fuel rods 3 will be described with reference to FIG. 17. Each of the long and short fuel rods 2 and 3 constituted by partially filling a cylindrical cladding tube 11 with a plurality of fuel pellets 10 and then sealing the upper and lower ends of the cladding tube 11 with an upper end plug 12 and a lower end plug 13. The upper portion above the active portion filled with the fuel pellets 10 is provided with a space of an approximately 40 cm length called gas plenum 14 to reduce a rise in the inner pressure due to nuclear fission product gas.
** 2 A spring 15 is inserted in the gas plenum 14 to fix the fuel pellets during transport of the fuel assembly. Each long fuel rod 2 is filled with the fuel pellets to the lower end portion, however each short fuel rod 3 is provided with another gas plenum 14 also at the lower portion and the position of the fuel pellets is fixed with a support member (not shown).
** 3 In each of the usual fuel rods, since the gas plenum 14 is positioned in the portion above the active portion, it does not affect the core characteristic such as thermal margin, shut down margin and the like. However, in the case where the short fuel rod is provided with the gas plenum above the active portion, the gas plenum is located in a position corresponding to the active portion of the long fuel rod, and therefore, the influence on the core characteristic can not be ignored.
** 4 Since use of the short fuel rods 3 originally reduces the fuel inventory per fuel assembly, the cost of the fuel cycle is increased and economy becomes worse. Moreover, since each short fuel rod has less coolant in the axial position of the gas plenum than at the upper portion thereof, the shut down margin becomes worse. In the example shown in FIG. 17, such a worsening of the shut down margin is minimized by partially distributing the gas plenum 14 to the lower end portion of the short fuel rod.
** 5 Consequently, in order to extend the active fuel length for improving fuel economy, the gas plenum 14 must be shortened, because the entire length of each short fuel rod 3 can not be made longer from the viewpoint of a change for the worse in the stability due to an increase in pressure loss, and the maintenance of the relation of positions between the upper ends of the short fuel rods 3 and the spacers 8. In addition, the gas plenum 14 is preferably as short as possible for enhancement of the shut down margin. However, the gas plenum 14 can not be easily shortened due to the primary object of the gas plenum which is to reduce the rise in inner pressure due to nuclear fission product gas.
The fuel assembly for high burn-up as constituted above has the following features as compared with the fuel assembly for low burn-up in the prior art disclosed in JPA 296192 (1990).
That is, a high enrichment of fuel is necessary to achieve the high burn-up, however the peaking of axial power which results from the void distribution is increased more than ever thereby. In addition, since more various histories of fuels exist together with different periods of staying in the core, an increase in the peaking of radial power is also brought about.
As a result, the thermal margins of the maximum linear heat generation rate, the minimum critical power ratio and the like are reduced. In order to improve such disadvantages, the number of the fuel rods in the fuel assembly 1 shown in FIG. 16 is increased by changing the arrangement of the fuel rods of 8 rows and 8 columns to that of 9 rows and 9 columns.
However, such an increase in the number of the fuel rods causes an increase in pressure loss, thereby impairing the stability of the nuclear reactor. For this reason, in the fuel assembly 1 shown in FIG. 16, a part of the long fuel rods are made shorter in length into the short fuel rods 3 which are used to thereby enlarge the upper flow path for fuel which is greater in pressure loss because of the two-phase flows of coolant, thereby canceling the increase in pressure loss due to the increase in the number of the fuel rods. The length of each short fuel rod 3 amounts to approximately 2/3 that of each long fuel rod 2, as apparent from FIGS. 17(A) and (B) .
In the meantime, if the power becomes excessive, heat transfer from the fuel rods to the coolant changes from efficient nucleate boiling to inefficient film boiling, and the power of the fuel assembly, when such a boiling transition occurs, is a critical power.
There is a high possibility that the boiling transition occurs at the upper portions of the fuel rods. Each short fuel rod 3 is positioned at the place where the cooling efficiency of the fuel rods is lower, thereby causing an increase in the minimum critical power ratio.
Each short fuel rod 3 further has a function of enhance the shut down margin. When the reactor is shut down, the neutron flux forms a peak in a position 1/4 to 1/3 down the entire length of the core from its upper end.
When the reactor is shut down, the coolant functions as a neutron absorber because of a lower temperature and higher density, and therefore, the reduction of the number of the fuel rods and the increase in the amount of the coolant in the axial upper portion thereof enables the shut down margin to be enhanced.
In the fuel assembly 1 composed of the long fuel rods 2 and the short fuel rods 3, as described above, since the number of the fuel rods is different in the axial upper region where the short fuel rods 3 do not exist (the section taken along line B--B of FIG. 16(B)) and the axial lower region where the short fuel rods 3 exist (the section taken along line C--C of FIG. 16(C)), the characteristic of the reactivity during the operation is greatly different at the upper and lower parts of the fuel assembly.
The high-energy neutron produced by nuclear fission is easily slow down at the upper part where the ratio of moderator to fuel is large, and therefore, the infinite neutron multiplication factor at the upper part is larger than that at the lower part.
While the boiling water reactor originally easily produces a power peak at the lower part during the power operation because of void distribution, in the core loaded with the fuel assembly 1 including the short fuel rods 3, the difference in the ratio of moderator to fuel between the upper and lower parts due to the difference in the number of the fuel rods reduce the power peak, thereby providing the preferable effect of the axial power distribution being flattened for a long period of burn-up.
However, at the beginning of the fuel lifetime, there is a problem that the difference in the number of the fuel rods at the upper and lower parts increases the power peaking occurring at the lower part. In the fuel assemblies used for a boiling water reactor, any burnable poisonous matter such as gadolinia is generally mixed in a part of the fuel rods for control of the reactivity.
This lowers the infinite neutron multiplication factor at the beginning of burn-up to thereby flatten the change of the excess reactivity in the core and enhance the operating and safe performance of the reactor. The amount of control of the reactivity at the beginning of burn-up due to gadolinia is substantially proportional to the number of the fuel rods containing gadolinia, and the period when the control of the reactivity is continued is substantially proportional to the concentration of gadolinia.
However, in the fuel assembly 1 including the short fuel rods 3 as shown in FIG. 16, even if the number of the fuel rods containing gadolinia is equal in the upper and lower parts thereof, there is a difference in the poisonous effect of gadolinium due to the difference in the ratio of moderator to fuel, and consequently, the amount of control of the reactivity at the beginning of burn-up is larger in the upper part, where the number of the fuel rods per section of the fuel assembly is less and the amount of moderator is more, than in the lower part.
As a result, particularly at the beginning of the operating cycle of the reactor, the infinite neutron multiplication factor in the upper part becomes smaller than that in the lower part and the peaking of power in the lower part of the core increases.
As an example, the infinite neutron multiplication factors, at the time the void fraction is 40%, of the fuel assembly 1 shown in FIG. 16, in the case where the average enrichment of fuel assembly is approximately 4% and gadolinia is not contained, are shown with curves "a" and "b" in FIG. 18. Curves "a" and "b" show the infinite neutron multiplication factors at the lower part and the upper part, respectively.
Further, since the void fraction in the core is smaller than 40% at the lower part and larger than 40% at the upper part, the comparison of the infinite neutron multiplication factors in the void fraction at each of the upper and lower parts is more precise. However, since the relation of the relative difference of the infinite multiplication factors between the upper and lower parts is important here, a comparison is made with the same void fraction.
As shown in FIG. 18, the infinite neutron multiplication factors are larger at the upper part (curve b) than at the lower part (curve a), and the difference therebetween is at a maximum at the beginning of the burn-up and reduces along with the increase of the burn-up. In general, the axial power distribution of the boiling water reactor has a lowermost peak at the beginning of the operating cycle and gradually shifts upwards to the end of the operating cycle, since the burn-up at the lower part proceeds earlier than at the upper part during burn-up.
The difference between the infinite neutron multiplication factors at the upper and lower parts shown with curves a and b in FIG. 18, is preferable to rectify the change in the power distribution for the burn-up as described above and to provide a flat axial power distribution throughout the operating cycle.
On the contrary, the change in the infinite neutron multification factors for burn-up in the case where gadolinia of a concentration of 3.5% is added to 14 long fuel rods 2 over the entire length is shown with curves c and d in FIG. 18. Curves c and d show the infinite multiplication factors at the lower and upper parts, respectively.
Since the capability to control the reactivity due to gadolinia is larger at the upper part than at the lower part, the infinite neutron multiplication factors at the upper and lower parts are reversed at the beginning of burn-up, and an increase in the downward peak at the beginning of the operating cycle is caused.
For the characteristics of the cores loaded with such fuel assemblies, (A) axial (and radial) power peaking, (B) maximum linear heat generating rate, and respective axial power distributions (C) at the beginning of the operating cycle and (D) at the end of the operating cycle are shown in FIG. 19 concerning a core loaded with a first fuel assembly composed of 66 long fuel rods 2 and 8 short fuel rods 3, and a core loaded with a second fuel assembly composed only of 74 long fuel rods 2.
In any fuel assembly, all the long fuel rods 2 are provided with regions of natural uranium at the upper and lower ends thereof, and gadolinia with a concentration of 3.5% is added to 14 long fuel rods 2 among them in their inner regions except the regions of natural uranium.
In FIG. 19, the respective axial power peakings of the first and second fuel assemblies are shown with curves e and f, the respective maximum linear heat generating rate with curves g and h, and the respective axial power distributions with curves i and k and with curves j and l.
The core loaded with the first fuel assemblies, the axial power distribution of which becomes a downward peak from the beginning to the middle of the operating cycle, has an increased axial power peaking as compared with the core loaded with the second fuel assemblies, and the maximum linear heat generating rate increases at the beginning of the operating cycle by the maximum 0.6 kw/ft. At the end of the operating cycle, the differences in the axial power distributions and also in the maximum linear heat generating rate between the first and second fuel assemblies, are small.
Hereupon, the features of the axial power distribution described above, have the effect of improving fuel economy, which is described, for example, in JPA-296192(1990). That is, the operation with a downward peak of axial power distribution from the beginning to the middle of the operating cycle increases the average void fraction of the core, and particularly, hardens neutron energy spectrum in the upper part of the core.
As a result, the production of plutonium is promoted, and the axial power distribution becomes an upward peak at the end of the operating cycle, causing the plutonium accumulated in the upper part to be burned up effectively. Such an effect is called as spectral shift effect.
In the characteristic of the infinite neuron multiplication factor in FIG. 18 in the fuel assembly 1 shown in FIG. 16, the difference in the infinite neutron multiplication factors between the upper and lower parts resulting from the difference in the number of the fuel rods rectifies the downward peak at the beginning of the operating cycle and flattens the axial power distribution, thereby reducing spectral shift effect.
In the manufacturing process of the fuel rods for the prior embodiment described above the fuel pellets are filled in the cladding tube one by one, however the more the kinds of the fuel pellet enrichments to be used to fill the tube are, the more complicated the manufacturing process becomes.
** 6 Besides, after the cladding tube is filled with the fuel pellets, a checking operation is made to see if they have filled the tube correctly; however, the more the kinds of the fuel pellet enrichment are, the more complicated the checking operation becomes. Consequently, according to the circumstances, such a checking operation has the possibility of increasing the manufacturing cost.
** 7 In addition, the critical power is increased by providing short fuel rods in place of the long fuel rods in the position where the cooling efficiency is worst; however, there is the possibility that a boiling transition occurs in the long fuel rods. In order to further improve upon such a disadvantage, making the enrichment at the upper portion of the long fuel rods lower is conceivable, however, this results in production of the distribution of the enrichment, which, in turn, brings about the complicated manufacturing process of the fuel rods, as described above.
On the other hand, for example, JPA-296192(1990) discloses the fact that two types of fuel assemblies, type 1 having more fuel rods containing gadolinia and type 2 having less fuel rods containing gadolinia, are previously provided and the ratio of the number of the type 1 and type 2 assemblies is varied as occasion demands, thereby coping with fluctuation in any period of the operating cycle, so-called the technique regarding a 2 type burnable poison design fuel assembly core (hereinafter referred to as 2 stream fuel assembly core).
That is, the fluctuation in length of the current or previous operating cycle often compels a change from the original plan in the number of assemblies where the fuels are to be replaced. In such a case, if the number of assemblies to be replaced decreases compared with the plan, more type 1 fuel assemblies are loaded, and if the number of assemblies to be replaced increases compared with the plan, more type 2 fuel assemblies are loaded. This allows the excess reactivity of the core at the beginning of the cycle to be set to a suitable range of 1-2% .DELTA.k.
Generally, in such a 2 stream fuel assembly core, type 1 fuel assemblies with a higher number of fuel rods containing gadolinia and a smaller infinite neutron multiplication factor are arranged in the center of the core, and type 2 fuel assemblies are arranged outside the center of the core.
Since the power distribution in the radial direction of the core is higher in the central portion and lower outside the central portion, the two types of fuel assemblies with a different number of fuel rods containing gadolinia are arranged as described above, so that the power distribution in the radial direction is flattened and the maximum linear heat generating rate and minimum critical power ratio are improved.
However, in such a core, there are some cases where the maximum linear heat generating rate appears in the fuel assemblies arranged outside the center of the core. In general, since higher burn-up results in higher enrichment, the infinite neutron multiplication factor after gadolinia burns out becomes larger and, on the other hand, the number of the fuel rods containing gadolinia per fuel assembly increases: accordingly, the infinite neutron multiplication factor at the beginning of burn-up decreases.
Therefore, at the beginning of the operating cycle, it is easy for the maximum linear heat generating rate to appear in the fuel in the second cycle after loading with the greatest infinite neutron multiplication factor. The concentration of gadolinia is set to burn out at the end of the once operating cycle; however, since the power is lower outside the center of the core than in the central portion and the speed of burn-up is slow, there is a possibility that the infinite neutron multiplication factor reaches a peak just in the fuel in the second cycle after loading arranged outside the center of the core.
Moreover, when the burn-up proceeds with a downward power peak, at the end of operating cycle the infinite multification factor at the upper part becomes larger than that at the lower part in the central portion, however the downward power peak remains unchanged since the burn-up is less outside the center of the core as compared with that at the center of the core. Accordingly, in the core with a 2-stream fuel assembly, different axial designs are required for each fuel assembly corresponding to the radial position where new fuel assemblies are loaded.
Further, in the case where a transition is made from an equilibrium core loaded only with fuel assemblies for lower burn-up having low enrichment to an equilibrium core loaded only with fuel assemblies for high burn-up having high enrichment, the fuel assemblies for low burn-up are removed and the fuel assemblies for high burn-up are loaded successively with every exchange of fuel.
In such a first or second transition cycle, since the fuel assemblies for high burn-up having high enrichment are loaded in the core in a smaller ratio than those for low burn-up having low enrichment, maximum radial power peakings occur in the fuel assemblies for high burn-up.
In addition, the higher the enrichment of fuel is, the more downward the peak of the axial power distribution thereof becomes. Therefore, there is the problem that in the transition cycle, it is easy for the maximum linear heat generating rate to increase as compared with that of the equilibrium core.